1. Field of the Invention
The present invention relates to a surface emitting laser. Particularly the invention relates to a distributed feedback type photonic crystal surface emitting laser that can align polarization of an output beam in a one-dimensional direction while emitting a laser beam in a single transverse mode.
2. Description of the Related Art
One of the features of the surface emitting laser, which is one of a semiconductor laser, is that light is emitted in a perpendicular direction or an oblique direction with respect to a substrate. Recently there is studied the distributed feedback (DFB) type surface emitting laser that emits a laser beam, which resonates in an in-plane direction of the substrate, to an outside of the plane with a diffraction grating. Hereinafter, the distributed feedback type surface emitting laser is abbreviated to a DFB type surface emitting laser.
Sakai et al. (IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, vol. 23, 1335 (2005)) disclose a DFB type surface emitting laser in which a two-dimensional photonic crystal, in which a diffraction grating is two-dimensionally formed, is used.
In the two-dimensional photonic crystal disclosed in Sakai et al., two primitive translation vectors have the same length. The length of the primitive translation vector is equal to λ/neff where λ denotes an oscillation wavelength and neff denotes an effective refractive index determined by the resonance mode. Since the two-dimensional photonic crystal in which the two primitive translation vectors have the same length forms a two-dimensional resonance mode, the DFB type surface emitting laser can operate in a single transverse mode regardless of a size of an emission area.
Further, since the primitive translation vector has the length λ/neff, the two-dimensional photonic crystal acts as a second-order diffraction grating. A diffraction perpendicular to the plane of the two-dimensional photonic crystal layer is generated by the first-order diffraction, and an in-plane diffraction is generated by the second-order diffraction.
Therefore, the laser light amplified by the in-plane diffraction is perpendicularly emitted by the first-order diffraction. In the perpendicularly-emitted laser beam, polarization reflects two-dimensional resonance, namely, the polarization includes two-dimensional vector components. Sakai et al. discloses an azimuthally polarized beam.
The reason the polarization direction becomes two-dimensional will be described in the following paragraphs by taking the two-dimensional photonic crystal of the related art disclosed in Sakai et al. as an example.
FIGS. 9A to 9C are three-dimensional schematic diagrams illustrating a reciprocal lattice space of the two-dimensional photonic crystal of a related art in which lattice points are arrayed into a square lattice. The numerals X1 and X2 designate primitive translation vectors in the reciprocal lattice space.
The diffraction of a TE-polarized wave vector k1 travelling in an X1 direction will be described with reference to FIG. 9A. The wave vector k1 becomes a diffracted wave k1′ in the perpendicular direction by the first-order diffraction. Since the polarization is maintained before and after the diffraction, the polarization direction of the diffracted wave k1′ is perpendicular to both the X1 direction and the k1′ direction. The polarization is expressed by a dotted-line arrow. At the same time, a diffracted wave k1″ travelling in a direction opposite to the diffracted wave k1 by 180 degrees is also generated by second-order diffraction, but the diffracted wave k1″ is not involved in the polarization of the light emitted in the perpendicular direction. The diffracted wave k1″ contributes to an amplification effect in a gain region.
Similarly the diffraction of a wave vector k2 travelling in an X2 direction will be described with reference to FIG. 9B. The wave vector k2 becomes a diffracted wave k2′ in the perpendicular direction by the first-order diffraction. The polarization direction is maintained, and the diffracted wave k2′ has the polarization oscillating in a direction perpendicular to both the X1 direction and the k2′ direction. At the same time, a diffracted wave k2″ travelling in a direction opposite to the diffracted wave k2 by 180 degrees is also generated by the second-order diffraction.
Since the first-order diffraction of the diffracted wave k1′ and the first-order diffraction of the diffracted wave k2′, which contribute to the perpendicular emission, are simultaneously generated, the diffracted wave k1′ and the diffracted wave k2′ are coupled in a wave k′ emitted in the perpendicular direction as illustrated in FIG. 9C. That is, the wave k′ becomes the polarization in which the two-dimensional components are combined.